Highly Luminescent Nanocrystals From Removal of Impurity Atoms Residual From Ion‐Exchange Synthesis

Abstract

Ion exchange of nanocrystals has the potential to emerge as an alternative to conventional routes for synthesis of ionic nanocrystals. [1][2][3][4][5][6][7][8][9] The facile ability to replace all cations of a nanocrystal with another cation, while preserving size and shape, allows us to employ nanocrystals as templates for the fabrication of other nanocrystals of interest.[6] Such a templated synthesis strategy is especially useful when the chemistry or crystallographic phase of the target nanocrystals is difficult to access via hot-injection methods. For instance, we recently showed [10] that Cu I sulfide quantum dots prepared by hot injection mostly result in the highly Cu-deficient djurleite phase. [11,12] The stoichiometric chalcocite phase is achievable, however, by room-temperature cation exchange of template CdS quantum dots with Cu + ions. Cation exchange holds particular promise for the fabrication of multicomponent heterostructured nanocrystals, [6,9] which allow independent tunability of electron and hole wavefunctions, but present potential synthetic challenges due to their greater structural complexity. Here, it is advantageous that the anionic framework of the heterostructure is maintained during cation exchange, allowing structural preservation of interfaces and junctions that define the electronic band alignment within the heterostructure. This has made possible the design and templated fabrication of novel semiconductor heterostructures that can range [13,14] from type-I, with high quantum yield emission useful for imaging and light-emitting diodes, [15,16] to type-II, which allow charge separation for photovoltaic and photocatalytic applications. [9,17] However, the ion exchange technique has been found to present a severe drawback: it results in nanocrystals with poor optoelectronic properties.[1] This is clear from a quantitative comparison of the optical properties of nanocrystals obtained from cation exchange with those prepared by standard hot injection for the model CdSe/CdS dot/rod heterostructure. [13][14][15][16] In this work, we trace the cause of the poor optical properties of cation-exchange-obtained nanocrystals to chemical impurities on the few atom per nanocrystal level. We have also found a method to purify the nanocrystals of these detrimental impurities post exchange and achieve optical properties comparable to those of hot-injection synthesized nanocrystals.Hot-injection synthesis of CdSe/CdS dot/rods with a 3.9 nm dot yields highly photoluminescent nanorods with a quantum yield (QY) of over 55 %, enabled by the type-I band alignment. On the other hand, CdSe/CdS dot/rods obtained from room-temperature exchange of Cu 2 Se/Cu 2 S dot/rods with Cd 2+ (see the Supporting Information) show relatively negligible emission, that is, a quantum yield (QY) of 0.07 %, almost three orders of magnitude smaller. This is despite the fact that the nanorods prepared by the two methods possess similar heterostructure morphologies, especially seed sizes, and consequently identical excitonic stru...

Cation Exchange: A Versatile Tool for Nanomaterials Synthesis

“…Despite the low temperatures of these exchange reactions, the product nanoheterostructures are of high optical quality. 59 Nanocrystals possessing a single anion host lattice can also serve as synthons for the preparation of nanoheterostructures via cation exchange. Provided that reactant and product phases are immiscible, partial cation exchange of a single-phase nanocrystal can generate a heterostructured product.…”

Formation of Onion‐Like NiCo2S4 Particles via Sequential Ion‐Exchange for Hybrid Supercapacitors

“…In recent years, chemical transformation of inorganic solids has emerged as an attractive approach for nanostructure synthesis. Among different transformation methods, ion‐exchange has been proven very effective for structural and compositional transformation of nanomaterials . For example, hollow particles can be easily synthesized from solid precursors via an ion‐exchange process by nanoscale Kirkendall effect .…”

Tuning the Magnetic Properties of Metal Oxide Nanocrystal Heterostructures by Cation Exchange

“…Postsynthetic substitution reactions, including galvanic replacement and cation exchange, applied to colloidal nanocrystals, 1−14 represent a simple and versatile tool to achieve nanoarchitectures not readily accomplishable by other methods. The galvanic replacement is predominantly applied to form noble metal nanocrystals, such as Au nanocages and nanoboxes, by reacting solutions of appropriate salts with nanocrystals as “nanotemplates”.…”

“…1−7 The cation exchange is a similar process, however, usually applied to compound semiconductors. 8−14 By cation exchange procedures, e.g., nanorod superlattices of regularly spaced Ag 2 S quantum dots in CdS colloidal quantum rods, 9 nonepitaxial hybrid nanostructures with gold nanoparticle cores and CdS shells, 15 or branched nanocrystals with either CdSe or Cu 2- x Se central cores and Cu 2 S pods, 16 have been demonstrated. In contrast to the galvanic replacement where often substantial morphology changes are observed (e.g., hollow structures are obtained from solid nanoparticles) 3,4 during cation exchange the nanocrystal shape is nearly preserved.…”